The harvesting of RF energy transmitted through the air for use in powering electronic devices is extremely important in a number of fields, such as radio frequency identification (RFID) systems, security monitoring and remote sensing, among others. For example, RFID systems consist of a number of radio frequency tags or transponders (RFID tags) and one or more radio frequency readers or interrogators (RFID readers). The RFID tags typically include an integrated circuit (IC) chip, such as a complementary metal oxide semiconductor (CMOS) chip, and an antenna connected thereto for allowing the RFID tag to communicate with an RFID reader over an air interface by way of RF signals. In a typical RFID system, one or more RFID readers query the RFID tags for information stored on them, which can be, for example, identification numbers, user written data, or sensed data.
RFID tags can generally be categorized as either passive tags or active tags. Passive RFID tags do not have an internal power supply. Instead, the electrical current induced in the antenna of a passive RFID tag by the incoming RF signal from the RFID reader provides enough power for the IC chip or chips in the tag to power up and transmit a response. One passive tag technology, known as backscatter technology, generates signals by backscattering the carrier signal sent from the RFID reader. In another technology, described in U.S. Pat. Nos. 6,289,237 and 6,615,074, RF energy from the RFID reader is converted to a DC voltage by an antenna/matching circuit/charge pump combination. The DC voltage is then used to power a processor/transmitter/antenna combination that transmits information to the RFID reader at, for example, a different frequency. In either case, the area of the tag or silicon die is valuable, and therefore it is advantageous to make the most efficient use of the space thereon. In addition, it is known that multiple antennas can be used to generate a DC voltage from an RF signal. For example, U.S. Pat. No. 6,734,797 describes a tag that uses two dipole antennas where the greater of the energies produced from the two antennas is the one that is selected and used. This, however, is not the most efficient use of tag space since the energy for the “loser” antenna is not used. Thus, there is a need for an energy harvesting circuit that is able to employ multiple antennas to optimize the amount of energy that is harvested while at the same time making efficient use of tag space.